The present invention relates to a reflector antenna intended for use in the microwave band or the millimeter wave band. More particularly, the invention relates to an antenna device having excellent wide angle radiation characteristics.
FIG. 1 shows the arrangement of a Cassegrain antenna, which is an example of a conventional reflector antenna.
In FIG. 1, reference numeral 1 designates a main reflector, 2 a subreflector, 3 a primary radiator, and 4 a supporting strut for supporting the subreflector. A spherical wave radiated by the primary radiator 3 of the reflector antenna thus constructed is converted into a plane wave upon being reflected by the subreflector mirror 2 and the main reflector 1. The plane wave is radiated outwardly. In this operation, the plane wave 5 reflected by the main reflector 1 is scattered by the supporting strut 4. In this case, the scattered wave 6 is composed of the reflected wave from the surface and the diffracted wave from the edge.
The section, on the main reflector side, of the conventional supporting strut 4 may be circular as shown in FIG. 2B or rectangular as shown in FIG. 2C. Therefore, the scattered waves 6 formed by these supporting struts have the radiation patterns as follows. If it is assumed that, as shown in FIG. 2A, the supporting strut 4 forms an angle .theta. with the Z-axis when the plane wave 5 advances in the positive direction of the Z-axis, and if, as shown in FIG. 3A, the observation point P has the coordinates (H, .PHI.) in polar coordinates with the Z-axis as the polar axis, then the direction of advancement of the reflected wave due to the supporting strut 4 can be represented by the following expression (1): ##EQU1## where .psi. is the angle formed between the plane Z-X and a normal line extending from the point on a reflection surface 8 at which the plane wave 5 is applied.
The direction of the diffracted wave is in the form of a cone having an edge 7 as the central axis and having a half vertical angle .theta.. In the case of FIG. 2B, the supporting strut 4 has a curved surface which has a continuously changing value .psi.. Therefore, the reflected wave is radiated in the form of a circular cone which has the supporting strut 4 as its central axis and has a half vertical angle .theta.. On the other hand, in the case of the supporting strut 4 shown in FIG. 2C, a reflected wave is produced due to a reflection surface 8 where the value .psi. is 90 degrees and waves are refracted due to edges 7. The reflected wave is radiated in the negative direction along the Z axis (-Z), while the diffracted waves are radiated conically.
The radiation patterns of the scattered waves due to the conventional supporting struts shown in FIGS. 2B and 2C are as shown in FIGS. 3B and 3C, respectively. In FIGS. 3B and 3C, the higher the density of lines shown therein, the higher the field intensity level. As is apparent from these figures, the field intensity level is high in the direction of scattering, and it is not low even in the region where the value H is large. This degrades the wide angle radiation characteristics of the antenna. Therefore, the use of such an antenna may cause interference with other radio systems.
This difficulty may be eliminated by employing a technique whereby a microwave absorber is provided on the surface of the supporting strut, or a technique whereby, as shown in FIG. 4, metal plates 9 are arranged at a certain pitch on the supporting strut 4, or a technique whereby metal elements which are shorter than the wavelength employed are irregularly arranged on the supporting strut. However, the first technique is disadvantageous in that it is impossible for the microwave absorber to completely absorb the scattered waves, and it is rather difficult to provide such a material which has satisfactory weather-proof characteristics. The second technique is also disadvantageous in that a grating lobe is formed according to the pitch at which the metal plates are arranged. The third technique has a drawback that, although the reflected wave can be scattered, it is difficult to scatter the diffracted wave.